A wide variety of self-expandable prostheses is known in the art such as stents or embolic filters. Many such self-expandable prostheses are made from elastic materials such as stainless steel and conichrome™—a cobalt-chromium-nickel-molybdenum-iron alloy. Such devices are typically introduced percutaneously to the patient; for example, filtering devices that are percutaneously placed in the vena cava have been available for over thirty years. Such percutaneous methods typically rely on these elastic prostheses being radially compressed so as to adopt a low profile configuration within an introducer sheath. In more detail, the prosthesis is typically attached to a stylet housed within a cannula, the cannula passing through the lumen of the sheath. The prosthesis may be fixed to the stylet by way of an attachment hook or snare at the distal end of the stylet cooperating with a feature on the proximal end of the prosthesis. The stylet is withdrawn within the cannula so that the prosthesis is mounted adjacent the distal end of the cannula, whereupon the stylet is locked relative to the cannula so that axial movement is prohibited thereafter and the prosthesis is held securely in position. The cannula-stylet assembly is then retracted, pulling the attached prosthesis into the lumen of the introducer sheath. During this part of the procedure it is necessary to radially compress the prosthesis so that it will fit within the lumen of the introducer sheath. This is commonly performed by a loading device at the distal end of the introducer sheath.
It has been found that maintaining an elastic prosthesis in a stressed configuration, for example the low profile configuration within the sheath, leads to degradation of the prosthesis owing to mechanical stresses within the prosthesis structure. Therefore, it is preferable for the prosthesis to be delivered to the surgeon or medical technician in an unstressed configuration. Typically this is done by performing the locking and mounting steps outlined above, but leaving the attached prosthesis outside the distal end of the introducer sheath. A handle is provided at the proximal end of the cannula to allow the surgeon or medical technician to retract the cannula and pull the prosthesis within the sheath. The entire apparatus, including the loading device, is provided assembled and hermetically sealed to the surgeon or medical technician, so that loading of the prosthesis may be accomplished simply by retracting the handle.
It has been found during such loading operations that often the distal end of the prosthesis will protrude from the distal end of the introducer sheath after the loading operation has been performed. Further, where the prosthesis comprises barbs for engagement with body lumens, these are likely to catch the open end of the introducer sheath, requiring a surgeon or technician's direct intervention; thus, it may be impossible to complete the loading without loss of sterility. Further, the barbs may scratch the interior of the introducer sheath and produce debris. The resulting debris may potentially be fatal if it is carried away during the procedure by the blood stream and lodges in a sensitive part of the vasculature—for example the lungs, where it may cause a pulmonary embolism.
Thus, there exists a need for a loading device which ensures correct placement of the prosthesis within an introducer sheath and also substantially prevents damage to the introducer sheath during loading of the prosthesis.